Web coating apparatus

ABSTRACT

In a coating apparatus, the web is conveyed and supported on a rotating roll. Liquid coating material is applied by atomizing the material with air to create a mist adjacent to and contiguous with the supported web. A thin line of pressurized air from the discharge orifice of an air knife is directed toward the web on an angle against the direction of web travel and into a confined space away from the orifice to create a turbulent zone adjacent the web. Liquid coating material is metered under pressure into the turbulent zone through a slot of a chamber. The coating on the web is leveled by the same air stream in a second zone closely adjacent the air orifice.

[ Mar. 26, 1974 United States Patent [191 Damrau WEB COATING APPARATUS2,511,415 6/1950 Putnam et 91/53 2,789.922

. 4 1957 All 117 106 Inventor: Wayne A. Damrau, W15. Rapids, en

Wis.

Primary Examiner-Mervin Stein Assigneei Consolidated p AssistantExaminer-Leo Millstein Attorney, Agent, or Firm--Gary, Juettner. Pigott& Cullinan Wisconsin Rapids, Wis.

Jan. 26, 1972 [22] Filed:

[21] Appl. No.: 220,895

[57] ABSTRACT In a coating apparatus, the web is conveyed and supportedon a rotating roll. Liquid coating material is applied by atomizing thematerial with air to create a 50 m B1 81 e 4 B 7n m I L C St Ur .1] 2155 [58] Field of Search...........

mist adjacent to and contiguous with the supported Web. A thin line ofpressurized air from the discharge orifice of an air knife is directedtoward the web on an [5 6] References Cited UNITED STATES PATENTS .1. mtmm L m m n ufl" .u O m fimre aO S Or n hin hA mmnn m e ahe o LHLETHMPM2328254 674737257 999999999 11 l l l 1 l ll 8 0725258 1 l 1 9250820 569892903 552893690 933736670 v 232323 23 WEB COATING APPARATUSBACKGROUND OF THE INVENTION This invention relates to an apparatus forcoating webs and more particularly to an apparatus for applying a thincoat of a coating composition to a moving flexible web composed of paperor the like.

It has been customary heretofore to utilize so-called dip or applicatorroll devices with or without air or mechanical doctor blades to applyand spread a liquid composition onto one or both of the surfaces of amoving strip material or web. Coating compositions have also beenapplied to a moving web by pumping the composition through a slot in ahopper and onto the web surface. These methods have been satisfactory tothe extent that uniform and usable coatings have been achieved at normalcoat weights. At lower coat weights, such as those in the order of about0.5 to 1 pound of solids per ream, it becomes increasingly difficult tomaintain uniformity by these methods, and many proposals to applyuniform low weight coatings have been made. A particular difficulty inapplying light weight coatings with blade coaters is the possibility ofexposing prominent surface fibers. Attempts at applying coatings byspraying have been generally unsuccessful because of the necessity ofoverlapping the spray patterns, resulting in an uneven coating.

BRIEF DESCRIPTION OF THE INVENTION The coating device of the presentinvention comprises a low pressure liquid applicator in cooperation witha high velocity air knife, both located closely adjacent one another anddirected against the web along the width thereof. The air knife orificeis directed on an angle against the direction of web travel in such amanner as to cause a thin stream of air to converge and impinge the webalong a distinct zone immediately adjacent the orifice. The air isdeflected and becomes divergent and increasingly turbulent toward thearea of application after the first incidence of impingement and isconfined adjacent the web by a spacer between the air knife and theoutlet of the applicator. The liquid coating composition is introducedfrom a slot in the applicator toward the web and into the turbulent airfrom the air knife, such that the composition is at least partiallyatomized. A mist of coating material is thus deposited on the web, incontradistinction to direct liquid application of many prior artmethods. After deposition of the coating mist, the coated web passesthrough the zone of convergence of the air knife whereby the wet coatingon the web is leveled or skimmed.

While the apparatus of the present invention is useful in applying awide range of coating weights to a web, the primary advantages are twoin number, namely the ability to deposit thin or light weight coatingsand the ability to generate a relatively coarse or rough surface (on afine scale) when formulations containing substantial amounts of pigmentare applied. For example, the apparatus may be employed in theapplication of thin coats of relatively expensive adhesives and/or filmformers to a previously coated or uncoated web, thereby resulting inconsiderable savings of material. To this end a conventional coater maybe placed in series with the present apparatus for the purpose ofapplying coating of high solids content, followed by a thin finishovercoat from the presently described apparatus.

Another application may be in the manufacture of electrostatic paperswhere a relatively rough top coat on the sheet is desirable to give thesheet good tooth that is receptive to writing instruments.

Other advantages and objects of the present invention will becomeapparent from the following description of a preferred embodimentthereof, taken in connection with the appended claims and accompanyingdrawing.

BRIEF DESCRIPTION OF THE DRAWING The FIGURE illustrated in the drawingis a vertical sectional view of the novel web coating apparatus, saidapparatus being shown in connection with a web supported upon a roll.

DESCRIPTION OF THE PREFERRED EMBODIMENT In the preferred embodiment, theweb coating apparatus, shown generally at 10, is supported in a positionto operate in conjunction with a web 12 of paper or the like partiallywrapped around and carried upon a backing roll 14. The backing roll 14rotates about its axis in the direction indicated by the arrow tocontinuously move the web 12 in the same direction past the coatingapparatus and to enable application of a coating to the web in acontinuous fashion. The coating device is shown in isolation with thebacking roll, but it will be understood that the device may beincorporated di' rectly in the line of a web forming machine, such as apaper machine, or may be employed in a separate coating and finishingline. Also, it will be understood that related conventional equipment,such as driers and calendering rolls, have been omitted from the presentdescription as being unessential for a full and complete understandingof the present invention.

The coating device generally comprises an air impingement component orair knife 16 which cooperates with a liquid metering component 18located ahead of the air impingement component in terms of direction ofweb travel. The components 16 and 18 are pivotally mounted as anassembly and are separated by a spacer 20. The assembly is substantiallyenclosed at its sides by parallel side walls, such as 22 which alsoencompass the sides of the backing roll in the vicinity of application.The air outlet orifice 24 of the air knife 16 is directed inward towardthe supported web and toward the outlet orifice 26 of the meteringcomponent 18, and there is a substantially enclosed space between saidorifices defined by the spacer 20, side walls 22 and web 12, into whichair from the air knife is channeled and becomes turbulent. The turbulentair then impinges on liquid issuing from the orifice 26 of the meteringcomponent 18 and causes atomization of the coating liquid in an areaadjacent the moving web ahead of said orifice. The air knife 16 alsoserves the second function of skimming the wet coating on the web in avicinity immediately adjacent to its outlet orifice 24.

The air impingement component or air knife 16 may be adjustablysupported and more particularly comprises a partial cylindrical member28 defining a plenum chamber to which is connected a source ofpressurized pneumatic fluid (not shown), such as an air pressure tanksupplied by a pump. A pair of elongated converging lips 30 and 32 extendradially from the open side of the cylindrical member 28 to define thenarrow outlet orifice 24 that extends across substantially the entirewidth of the web 12 along a zone perpendicular to the sides of said web.One of the lips 32 may include a separate and replaceable end section34, which may comprise a flat outer surface 36 and a convexly curvedinterior surface 37, terminating in a sharp edge and partially definingthe air flow characteristics from the outlet orifice 24. The other lip30 comprises a flat interior surface 38, with the outer surface having atapered portion 42 defining a sharp edge.

The lips 30 and 32 are narrowly spaced from one another at the outlet 24and are closely spaced from the supported web 12 to cause a thin jet ofair to be directed toward the web surface. In order to define optimumair flow characteristics, the distance between the lips 30 and 38 at theoutlet 24 is in the order of about 20 to 30 thousandths of an inch. Theangle 41 of the air knife downward from horizontal toward the web ispreferably maintained from about 45 degrees to 5 3 degrees, and optimumresults are achieved at about 47 degrees. Also, the distance between theair orifice 24 and the web, indicated at 43, is important in definingproper air flow characteristics and should be maintained between aboutone-eighth to one sixty-fourth inches. In this manner, the air issuingfrom the orifice 24 will initially be convergent and will impinge theweb 12 along a sharp line to achieve a flexible skimming effect.

The external surface 36 of the lip section 34 is supported upon a flatsurface of the spacer 20, which may generally comprise a rectangular barextending across the width of the web. Preferably, the spacer 20 isadjustable relative to the air knife in a direction toward and away fromthe web, and the surface thereof facing the web and spaced therefrom mayhave a groove 45 therein. The groove 45 is preferably most sharplyrecessed immediately below the air outlet 24 to define an edge 44 evenwith the edge of the lip section 34. The part of the groove 45 adjacentthe liquid orifice 26 is more gradually curved to form a relativelythick edge 46, which projects slightly beyond the other edge 44.

The confined space between the groove 45 and the support web 12 forms achannel of fluid communication between the air orifice 24 and the liquidorifice 26 wherein the air stream, after its initial impingement uponthe web, may become divergent or turbulent before encountering thecoating liquid. In order to prevent excessive accumulation of thecoating material on the lips 30 and 32 of the air knife 16, it has beenfound that a thin, open end slot 48 between the lip section 38 and thespacer 20 facing the web will minimize this undesirable result. The slot48 is preferably in the order of about from one thirty-second toone-half inches in thickness and apparently causes minor air turbulencein a vicinity immediately below the air orifice 24, thereby minimizingspattering of liquid against the lip surfaces.

The liquid metering component 18 is connected to the side of the spacer20 opposite to that of the air knife 16 and comprises a flanged L-shapedchannel 50 secured from the spacer and carrying a corresponding reverseL-shaped blade support 52. The channel 50 and the support 52 togetherdefine a fluid tight chamber 53 for containing a liquid coatingcomposition, such as an aqueous mixture of a powdered mineral materialand an adhesive, which may comprise, for example, clay and starch. Aninlet 54 to the chamber 53 may be provided in the channel 50 throughwhich the desired coating mixture may be delivered under pressure from apump (not shown). A wide range of solids may be incorporated into thecoating material, such as between about 22 percent (or lower) to 37percent, and at viscosities ranging from at least 16 to 264 centipoisesat 20 rpm Brookfield.

A blade 56 having a tapered end is mounted on the support 52 and extendsacross the width of the web 12 in a plane substantially parallel toaplane tangential to the backing roll 14. The tapered end of the blade 56is preferably located in a position level with or behind the edge 46 ofthe spacer 20 at a distance of up to about three-sixteenths of an inch,said distance being indicated at 58. To achieve optimum results, theoffset spacing indicated at 58 is preferably adjusted to aboutone-eighth of an inch. The blade 56 is also adjustably spaced from thespacer 20 to define the liquid orifice 26, said orifice being separatedfrom the web by from three-sixteenths to three-eighths of an inch.

The metering component operates by means of maintaining positivepressure upon the liquid coating mixture within the chamber wherebyliquid is forced out of the orifice 26 toward the facing surface of theweb 12. In order to assure a uniform coating, the pressure within thechamber is maintained at a constant level within the range of about 0.1to 3 pounds per square inch. The liquid coating mixture is thereforeinjected into the turbulent atmosphere created by the air knife 16,which causes the coating to atomize and be deposited upon the web.

In the operation of the present apparatus, an excess of coating liquidis introduced through the orifice 26 and means are provided to collectand recirculate unused material back into the chamber 53. A barrier 60comprising a knife with a tapered edge extending closely adjacent thewidth of the web is spaced from and substantially parallel to the blade56, and is located ahead of the metering component 18 in terms of webtravel. The barrier 60 is also encompassed by the side walls 22 andprevents escape of atomized coating material, part of which is depositedon the web between the blade 56 and the barrier 60. A collecting chamberor separator pan and pump (not shown) may be located beneath theenclosed space between the blade 56 and the barrier 60, in order toreturn unused coating material to the chamber 53.

The operation of the coating apparatus and the method of the presentinvention will now be apparent. The coating apparatus is preferablysupported in the position shown in the drawing, that is, the air knifeis slanted downward about 47 degrees from horizontal, with its orificelocated below a horizontal plane passing through the axis of the backingroll 14. It will be obvious, however, in view of the atomization conceptby which the apparatus operates, that the apparatus may be located inother axial positions with respect to the backing roll 14.

In the operation, the air knife 16 is pressurized to create anon-turbulent zone of air impingement immediately adjacent the orifice24 thereof and a substantially turbulent zone adjacent the liquidorifice 26. Thereafter, a stream of liquid coating material is injectedthrough the orifice 26 into the turbulent air and not directly onto theweb 12. In this manner, the liquid is initially atomized to create aturbulent and homogeneous mist continguous with the moving web, wherebya film of substantially uniform and constant thickness is deposited onthe web between the orifice 26 and the barrier 60. The thus coated webis then immediately passed through the non-turbulent zone of the airknife 16, which serves to level or skim the coating without scrapingcoating material off of prominent surface fibers in the web. The web isthen dried in a conventional drier and may then be subjected to acalendering oper ation.

The apparatus herein described is extremely versatile in terms ofadjustability and may apply coat weights ranging from 0.5 to 5 poundsper ream and at web speeds in excess of 1,500 feet per minute. The coatweight may be varied by changing the pressure in th air impingementcomponent 16 or in the liquid chamber 53, by changing either or both ofthe orifice gaps 24 and 26, by adjusting the air knife angle 41, byincreasing or decreasing the distance between the apparatus and the roll14, or by varying the coating solids, or by combinations of any of theabove.

Another important feature of the present invention is that no parts ofthe coating apparatus contact the web. The coating is applied bydeposition of a mist of the liquid material onto the moving web, and thecoating is immediately leveled or skimmed by an air knife which is alsospaced from the web. The cooperation between the air impingementcomponent and the metering component is particularly advantageous inthat the air component serves the two-fold purpose of atomizing thecoating material and skimming the coated web. Also, the close proximityof the air knife to the metering device allows less time for moisture inthe coating to migrate into the web and also allows for convenientrecirculation of the unused coating liquid.

Other important advantages of the presently described coating device arethe ability to apply coatings of low solids content and to coverprominent surface fibers in the web. Since the apparatus does notcontact the web, it may be used to apply low coat weights onto adilitant wet coated surface without unduly disrupting the previous coat,and-without substantial increase in the drying load. For example, theprevious coat may be applied by a conventional blade coater, to beimmediately followed by a lighter coat from the apparatus of the presentinvention, in order to cover prominent surface fibers, or to utilize arelatively expensive but highly advantageous coating material.

Having thus described the invention, what is claimed 1. Apparatus forapplying coating material to a moving web comprising air impingementmeans spaced from said web for directing a stream of high velocity airtoward and across the web on an angle against the direction of movementthereof, and means for atomizing liquid coating material onto said web,said means comprising metering means spaced from said web for meteringliquid coating material toward and across said web, said metering meansbeing located ahead of said air impingement means with respect to webtravel, and means defining a substantially confined space between saidweb and said air impingement means and said metering means, said airimpingement means creating a first distinct zone of impingement withsaid web and becoming divergent and turbulent in said confined space toatomize said liquid coating material.

2. The apparatus of claim 1 wherein said air impingement means comprisesan air knife having a plenum chamber and a pair of lips converging fromsaid chamber and terminating in sharp edges adjacent said web.

3. The apparatus of claim 1 wherein said means defining a substantiallyconfined space comprises a wall disposed between said air impingementmeans and said metering means, said wall being spaced from said web.

4. The apparatus of claim 3 wherein said wall has a channel thereinextending across the width thereof.

5. The apparatus of claim 3 wherein said channel is located more closelyadjacent to the air impingement means than the metering means.

6. The apparatus of claim 3 wherein said wall is defined by a barbetween said air impingement means and said metering means, and saidmetering means comprises container means defining a chamber, means forsupplying liquid coating material to said chamber under pressure, and aknife extending from said container and spaced from said bar to definean outlet slot.

7. The apparatus of claim 6 wherein said blade is substantially evenwith, or is offset from the wall of said bar away from said web suchthat bar overhangs said blade.

8. The apparatus of claim 6 wherein a slot is provided between said airimpingement means and said wall, said slot being open toward said webbut otherwise being enclosed.

9. The apparatus of claim 7 wherein the degree of offset between saidblade and said wall is from about 0 to three-sixteenths inches, said airimpingement means is spaced from said web in the order of about fromoneeighth to one sixty-fourth inches and is directed along an angle inth order of about from 45 to 53 degrees from horizontal, and thepressure within said chamber is in the order of about from 0.] to 3pounds per square inch.

1. Apparatus for applying coating material to a moving web comprisingair impingement means spaced from said web for directing a stream ofhigh velocity air toward and across the web on an angle against thedirection of movement thereof, and means for atomizing liquid coatingmaterial onto said web, said means comprising metering means spaced fromsaid web for metering liquid coating material toward and across saidweb, said metering means being located ahead of said air impingementmeans with respect to web travel, and means defining a substantiallyconfined space between said web and said air impingement means and saidmetering means, said air impingement means creating a first distinctzone of impingement with said web and becoming divergent and turbulentin said confined space to atomize said liquid coating material.
 2. Theapparatus of claim 1 wherein said air impingement means comprises an airknife having a plenum chamber and a pair of lips converging from saidchamber and terminating in sharp edges adjacent said web.
 3. Theapparatus of claim 1 wherein said means defining a substantiallyconfined space comprises a wall disposed between said air impingementmeans and said metering means, said wall being spaced from said web. 4.The apparatus of claim 3 wherein said wall has a channel thereinextending across the width thereof.
 5. The apparatus of claim 3 whereinsaid channel is located more closely adjacent to the air impingementmeans than the metering means.
 6. The apparatus of claim 3 wherein saidwall is defined by a bar between said air impingement means and saidmetering means, and said metering means comprises container meansdefining a chamber, means for supplying liquid coating material to saidchamber under pressure, and a knife extending from said container andspaced from said bar to define an outlet slot.
 7. The apparatus of claim6 wherein said blade is substantially even with, or is offset from thewall of said bar away from said web such that bar overhangs said blade.8. The apparatus of claim 6 wherein a slot is provided between said airimpingement means and said wall, said slot being open toward said webbut otherwise being enclosed.
 9. The apparatus of claim 7 wherein thedegree of offset between said blade and said wall is from about 0 tothree-sixteenths inches, said air impingemeNt means is spaced from saidweb in the order of about from one-eighth to one sixty-fourth inches andis directed along an angle in th order of about from 45 to 53 degreesfrom horizontal, and the pressure within said chamber is in the order ofabout from 0.1 to 3 pounds per square inch.